Journal articles: 'Molecular qubits'

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Yamamoto, Satoru, Shigeaki Nakazawa, Kenji Sugisaki, Kazunobu Sato, Kazuo Toyota, Daisuke Shiomi, and Takeji Takui. "Adiabatic quantum computing with spin qubits hosted by molecules." Physical Chemistry Chemical Physics 17, no. 4 (2015): 2742–49. http://dx.doi.org/10.1039/c4cp04744c.

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CAO, WEN-ZHEN, LI-JIE TIAN, HUI-JUAN JIANG, and CHONG LI. "SINGLE QUBIT MANIPULATION IN HETERONUCLEAR DIATOMIC MOLECULAR SYSTEM." International Journal of Quantum Information 06, no. 06 (December 2008): 1223–30. http://dx.doi.org/10.1142/s0219749908004390.

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We propose a scenario to realize quantum computers utilizing heteronuclear diatomic rovibrational states as qubits. We focused on rovibrational qubits created by simple transform limited infrared laser pulse instead of using chirped pulse. Numerical calculations show that single qubit gate operation in the electronic ground state of LiH molecule can be obtained. We also discuss the effect of temperature on the initially rotational states, and a suitable experiment condition is indicated.

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Hastings, Matthew B., and Jeongwan Haah. "Dynamically Generated Logical Qubits." Quantum 5 (October 19, 2021): 564. http://dx.doi.org/10.22331/q-2021-10-19-564.

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We present a quantum error correcting code with dynamically generated logical qubits. When viewed as a subsystem code, the code has no logical qubits. Nevertheless, our measurement patterns generate logical qubits, allowing the code to act as a fault-tolerant quantum memory. Our particular code gives a model very similar to the two-dimensional toric code, but each measurement is a two-qubit Pauli measurement.

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Tahan, Charles. "Opinion: Democratizing Spin Qubits." Quantum 5 (November 18, 2021): 584. http://dx.doi.org/10.22331/q-2021-11-18-584.

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I've been building Powerpoint-based quantum computers with electron spins in silicon for 20 years. Unfortunately, real-life-based quantum dot quantum computers are harder to implement. Materials, fabrication, and control challenges still impede progress. The way to accelerate discovery is to make and measure more qubits. Here I discuss separating the qubit realization and testing circuitry from the materials science and on-chip fabrication that will ultimately be necessary. This approach should allow us, in the shorter term, to characterize wafers non-invasively for their qubit-relevant properties, to make small qubit systems on various different materials with little extra cost, and even to test spin-qubit to superconducting cavity entanglement protocols where the best possible cavity quality is preserved. Such a testbed can advance the materials science of semiconductor quantum information devices and enable small quantum computers. This article may also be useful as a light and light-hearted introduction to quantum dot spin qubits.

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Mani, Tomoyasu. "Molecular qubits based on photogenerated spin-correlated radical pairs for quantum sensing." Chemical Physics Reviews 3, no. 2 (June 2022): 021301. http://dx.doi.org/10.1063/5.0084072.

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Photogenerated spin-correlated radical pairs (SCRPs) in electron donor–bridge–acceptor (D–B–A) molecules can act as molecular qubits and inherently spin qubit pairs. SCRPs can take singlet and triplet spin states, comprising the quantum superposition state. Their synthetic accessibility and well-defined structures, together with their ability to be prepared in an initially pure, entangled spin state and optical addressability, make them one of the promising avenues for advancing quantum information science. Coherence between two spin states and spin selective electron transfer reactions form the foundation of using SCRPs as qubits for sensing. We can exploit the unique sensitivity of the spin dynamics of SCRPs to external magnetic fields for sensing applications including resolution-enhanced imaging, magnetometers, and magnetic switch. Molecular quantum sensors, if realized, can provide new technological developments beyond what is possible with classical counterparts. While the community of spin chemistry has actively investigated magnetic field effects on chemical reactions via SCRPs for several decades, we have not yet fully exploited the synthetic tunability of molecular systems to our advantage. This review offers an introduction to the photogenerated SCRPs-based molecular qubits for quantum sensing, aiming to lay the foundation for researchers new to the field and provide a basic reference for researchers active in the field. We focus on the basic principles necessary to construct molecular qubits based on SCRPs and the examples in quantum sensing explored to date from the perspective of the experimentalist.

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Xue, Xiao, Maximilian Russ, Nodar Samkharadze, Brennan Undseth, Amir Sammak, Giordano Scappucci, and Lieven M. K. Vandersypen. "Quantum logic with spin qubits crossing the surface code threshold." Nature 601, no. 7893 (January 19, 2022): 343–47. http://dx.doi.org/10.1038/s41586-021-04273-w.

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AbstractHigh-fidelity control of quantum bits is paramount for the reliable execution of quantum algorithms and for achieving fault tolerance—the ability to correct errors faster than they occur1. The central requirement for fault tolerance is expressed in terms of an error threshold. Whereas the actual threshold depends on many details, a common target is the approximately 1% error threshold of the well-known surface code2,3. Reaching two-qubit gate fidelities above 99% has been a long-standing major goal for semiconductor spin qubits. These qubits are promising for scaling, as they can leverage advanced semiconductor technology4. Here we report a spin-based quantum processor in silicon with single-qubit and two-qubit gate fidelities, all of which are above 99.5%, extracted from gate-set tomography. The average single-qubit gate fidelities remain above 99% when including crosstalk and idling errors on the neighbouring qubit. Using this high-fidelity gate set, we execute the demanding task of calculating molecular ground-state energies using a variational quantum eigensolver algorithm5. Having surpassed the 99% barrier for the two-qubit gate fidelity, semiconductor qubits are well positioned on the path to fault tolerance and to possible applications in the era of noisy intermediate-scale quantum devices.

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Gidney, Craig, Michael Newman, and Matt McEwen. "Benchmarking the Planar Honeycomb Code." Quantum 6 (September 21, 2022): 813. http://dx.doi.org/10.22331/q-2022-09-21-813.

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We improve the planar honeycomb code by describing boundaries that need no additional physical connectivity, and by optimizing the shape of the qubit patch. We then benchmark the code using Monte Carlo sampling to estimate logical error rates and derive metrics including thresholds, lambdas, and teraquop qubit counts. We determine that the planar honeycomb code can create a logical qubit with one-in-a-trillion logical error rates using 7000 physical qubits at a 0.1% gate-level error rate (or 900 physical qubits given native two-qubit parity measurements). Our results cement the honeycomb code as a promising candidate for two-dimensional qubit architectures with sparse connectivity.

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Bravyi, Sergey, Ruslan Shaydulin, Shaohan Hu, and Dmitri Maslov. "Clifford Circuit Optimization with Templates and Symbolic Pauli Gates." Quantum 5 (November 16, 2021): 580. http://dx.doi.org/10.22331/q-2021-11-16-580.

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The Clifford group is a finite subgroup of the unitary group generated by the Hadamard, the CNOT, and the Phase gates. This group plays a prominent role in quantum error correction, randomized benchmarking protocols, and the study of entanglement. Here we consider the problem of finding a short quantum circuit implementing a given Clifford group element. Our methods aim to minimize the entangling gate count assuming all-to-all qubit connectivity. First, we consider circuit optimization based on template matching and design Clifford-specific templates that leverage the ability to factor out Pauli and SWAP gates. Second, we introduce a symbolic peephole optimization method. It works by projecting the full circuit onto a small subset of qubits and optimally recompiling the projected subcircuit via dynamic programming. CNOT gates coupling the chosen subset of qubits with the remaining qubits are expressed using symbolic Pauli gates. Software implementation of these methods finds circuits that are only 0.2% away from optimal for 6 qubits and reduces the two-qubit gate count in circuits with up to 64 qubits by 64.7% on average, compared with the Aaronson-Gottesman canonical form.

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Bahari, Iskandar, Timothy P. Spiller, Shane Dooley, Anthony Hayes, and Francis McCrossan. "Collapse and revival of entanglement between qubits coupled to a spin coherent state." International Journal of Quantum Information 16, no. 02 (March 2018): 1850017. http://dx.doi.org/10.1142/s021974991850017x.

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We extend the study of the Jayne–Cummings (JC) model involving a pair of identical two-level atoms (or qubits) interacting with a single mode quantized field. We investigate the effects of replacing the radiation field mode with a composite spin, comprising [Formula: see text] qubits, or spin-1/2 particles. This model is relevant for physical implementations in superconducting circuit QED, ion trap and molecular systems. For the case of the composite spin prepared in a spin coherent state, we demonstrate the similarities of this set-up to the qubits-field model in terms of the time evolution, attractor states and in particular the collapse and revival of the entanglement between the two qubits. We extend our analysis by taking into account an effect due to qubit imperfections. We consider a difference (or “mismatch”) in the dipole interaction strengths of the two qubits, for both the field mode and composite spin cases. To address decoherence due to this mismatch, we then average over this coupling strength difference with distributions of varying width. We demonstrate in both the field mode and the composite spin scenarios that increasing the width of the “error” distribution increases suppression of the coherent dynamics of the coupled system, including the collapse and revival of the entanglement between the qubits.

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Koiller, Belita, Xuedong Hu, Rodrigo B. Capaz, Adriano S. Martins, and Sankar Das Sarma. "Silicon-based spin and charge quantum computation." Anais da Academia Brasileira de Ciências 77, no. 2 (June 2005): 201–22. http://dx.doi.org/10.1590/s0001-37652005000200002.

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Silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideal candidates for qubits in such proposals due to the relatively long spin coherence times. For these spin qubits, donor electron charge manipulation by external gates is a key ingredient for control and read-out of single-qubit operations, while shallow donor exchange gates are frequently invoked to perform two-qubit operations. More recently, charge qubits based on tunnel coupling in P+2 substitutional molecular ions in Si have also been proposed. We discuss the feasibility of the building blocks involved in shallow donor quantum computation in silicon, taking into account the peculiarities of silicon electronic structure, in particular the six degenerate states at the conduction band edge. We show that quantum interference among these states does not significantly affect operations involving a single donor, but leads to fast oscillations in electron exchange coupling and on tunnel-coupling strength when the donor pair relative position is changed on a lattice-parameter scale. These studies illustrate the considerable potential as well as the tremendous challenges posed by donor spin and charge as candidates for qubits in silicon.

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11

Groszkowski, Peter, and Jens Koch. "Scqubits: a Python package for superconducting qubits." Quantum 5 (November 17, 2021): 583. http://dx.doi.org/10.22331/q-2021-11-17-583.

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scqubits is an open-source Python package for simulating and analyzing superconducting circuits. It provides convenient routines to obtain energy spectra of common superconducting qubits, such as the transmon, fluxonium, flux, cos(2ϕ) and the 0-π qubit. scqubits also features a number of options for visualizing the computed spectral data, including plots of energy levels as a function of external parameters, display of matrix elements of various operators as well as means to easily plot qubit wavefunctions. Many of these tools are not limited to single qubits, but extend to composite Hilbert spaces consisting of coupled superconducting qubits and harmonic (or weakly anharmonic) modes. The library provides an extensive suite of methods for estimating qubit coherence times due to a variety of commonly considered noise channels. While all functionality of scqubits can be accessed programatically, the package also implements GUI-like widgets that, with a few clicks can help users both create relevant Python objects, as well as explore their properties through various plots. When applicable, the library harnesses the computing power of multiple cores via multiprocessing. scqubits further exposes a direct interface to the Quantum Toolbox in Python (QuTiP) package, allowing the user to efficiently leverage QuTiP's proven capabilities for simulating time evolution.

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Yirka, Justin, and Yiğit Subaşı. "Qubit-efficient entanglement spectroscopy using qubit resets." Quantum 5 (September 2, 2021): 535. http://dx.doi.org/10.22331/q-2021-09-02-535.

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One strategy to fit larger problems on NISQ devices is to exploit a tradeoff between circuit width and circuit depth. Unfortunately, this tradeoff still limits the size of tractable problems since the increased depth is often not realizable before noise dominates. Here, we develop qubit-efficient quantum algorithms for entanglement spectroscopy which avoid this tradeoff. In particular, we develop algorithms for computing the trace of the n-th power of the density operator of a quantum system, Tr(ρn), (related to the Rényi entropy of order n) that use fewer qubits than any previous efficient algorithm while achieving similar performance in the presence of noise, thus enabling spectroscopy of larger quantum systems on NISQ devices. Our algorithms, which require a number of qubits independent of n, are variants of previous algorithms with width proportional to n, an asymptotic difference. The crucial ingredient in these new algorithms is the ability to measure and reinitialize subsets of qubits in the course of the computation, allowing us to reuse qubits and increase the circuit depth without suffering the usual noisy consequences. We also introduce the notion of effective circuit depth as a generalization of standard circuit depth suitable for circuits with qubit resets. This tool helps explain the noise-resilience of our qubit-efficient algorithms and should aid in designing future algorithms. We perform numerical simulations to compare our algorithms to the original variants and show they perform similarly when subjected to noise. Additionally, we experimentally implement one of our qubit-efficient algorithms on the Honeywell System Model H0, estimating Tr(ρn) for larger n than possible with previous algorithms.

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13

Tuna, Floriana. "Reaction: Molecular Spins as Qubits." Chem 6, no. 4 (April 2020): 799–800. http://dx.doi.org/10.1016/j.chempr.2020.03.017.

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Leviant, Peter, Qian Xu, Liang Jiang, and Serge Rosenblum. "Quantum capacity and codes for the bosonic loss-dephasing channel." Quantum 6 (September 29, 2022): 821. http://dx.doi.org/10.22331/q-2022-09-29-821.

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Bosonic qubits encoded in continuous-variable systems provide a promising alternative to two-level qubits for quantum computation and communication. So far, photon loss has been the dominant source of errors in bosonic qubits, but the significant reduction of photon loss in recent bosonic qubit experiments suggests that dephasing errors should also be considered. However, a detailed understanding of the combined photon loss and dephasing channel is lacking. Here, we show that, unlike its constituent parts, the combined loss-dephasing channel is non-degradable, pointing towards a richer structure of this channel. We provide bounds for the capacity of the loss-dephasing channel and use numerical optimization to find optimal single-mode codes for a wide range of error rates.

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15

Kurganskii, Ivan V., Evgeniya S. Bazhina, Alexander A. Korlyukov, Konstantin A. Babeshkin, Nikolay N. Efimov, Mikhail A. Kiskin, Sergey L. Veber, Alexey A. Sidorov, Igor L. Eremenko, and Matvey V. Fedin. "Mapping Magnetic Properties and Relaxation in Vanadium(IV) Complexes with Lanthanides by Electron Paramagnetic Resonance." Molecules 24, no. 24 (December 14, 2019): 4582. http://dx.doi.org/10.3390/molecules24244582.

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Vanadium(IV) complexes are actively studied as potential candidates for molecular spin qubits operating at room temperatures. They have longer electron spin decoherence times than many other transition ions, being the key property for applications in quantum information processing. In most cases reported to date, the molecular complexes were optimized through the design for this purpose. In this work, we investigate the relaxation properties of vanadium(IV) ions incorporated in complexes with lanthanides using electron paramagnetic resonance (EPR). In all cases, the VO6 moieties with no nuclear spins in the first coordination sphere are addressed. We develop and implement the approaches for facile diagnostics of relaxation characteristics in individual VO6 moieties of such compounds. Remarkably, the estimated relaxation times are found to be close to those of other vanadium-based qubits obtained previously. In the future, a synergistic combination of qubit-friendly properties of vanadium ions with single-molecule magnetism and luminescence of lanthanides can be pursued to realize new functionalities of such materials.

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Stajic, Jelena. "Molecular qubits that respond to light." Science 370, no. 6522 (December 10, 2020): 1286.7–1287. http://dx.doi.org/10.1126/science.370.6522.1286-g.

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Levi, Barbara Goss. "Making molecular-spin qubits more robust." Physics Today 69, no. 5 (May 2016): 17–21. http://dx.doi.org/10.1063/pt.3.3157.

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Affronte, Marco, Filippo Troiani, Alberto Ghirri, Stefano Carretta, Paolo Santini, Valdis Corradini, Raffael Schuecker, Chris Muryn, Grigore Timco, and Richard E. Winpenny. "Molecular routes for spin cluster qubits." Dalton Transactions, no. 23 (2006): 2810. http://dx.doi.org/10.1039/b515731e.

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Ferrando-Soria, Jesús, Samantha A. Magee, Alessandro Chiesa, Stefano Carretta, Paolo Santini, Iñigo J. Vitorica-Yrezabal, Floriana Tuna, et al. "Switchable Interaction in Molecular Double Qubits." Chem 1, no. 5 (November 2016): 727–52. http://dx.doi.org/10.1016/j.chempr.2016.10.001.

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Plachta, Stephen Z. D., Markus Hiekkamäki, Abuzer Yakaryılmaz, and Robert Fickler. "Quantum advantage using high-dimensional twisted photons as quantum finite automata." Quantum 6 (June 30, 2022): 752. http://dx.doi.org/10.22331/q-2022-06-30-752.

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Quantum finite automata (QFA) are basic computational devices that make binary decisions using quantum operations. They are known to be exponentially memory efficient compared to their classical counterparts. Here, we demonstrate an experimental implementation of multi-qubit QFAs using the orbital angular momentum (OAM) of single photons. We implement different high-dimensional QFAs encoded on a single photon, where multiple qubits operate in parallel without the need for complicated multi-partite operations. Using two to eight OAM quantum states to implement up to four parallel qubits, we show that a high-dimensional QFA is able to detect the prime numbers 5 and 11 while outperforming classical finite automata in terms of the required memory. Our work benefits from the ease of encoding, manipulating, and deciphering multi-qubit states encoded in the OAM degree of freedom of single photons, demonstrating the advantages structured photons provide for complex quantum information tasks.

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Bourassa, J. Eli, Rafael N. Alexander, Michael Vasmer, Ashlesha Patil, Ilan Tzitrin, Takaya Matsuura, Daiqin Su, et al. "Blueprint for a Scalable Photonic Fault-Tolerant Quantum Computer." Quantum 5 (February 4, 2021): 392. http://dx.doi.org/10.22331/q-2021-02-04-392.

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Photonics is the platform of choice to build a modular, easy-to-network quantum computer operating at room temperature. However, no concrete architecture has been presented so far that exploits both the advantages of qubits encoded into states of light and the modern tools for their generation. Here we propose such a design for a scalable fault-tolerant photonic quantum computer informed by the latest developments in theory and technology. Central to our architecture is the generation and manipulation of three-dimensional resource states comprising both bosonic qubits and squeezed vacuum states. The proposal exploits state-of-the-art procedures for the non-deterministic generation of bosonic qubits combined with the strengths of continuous-variable quantum computation, namely the implementation of Clifford gates using easy-to-generate squeezed states. Moreover, the architecture is based on two-dimensional integrated photonic chips used to produce a qubit cluster state in one temporal and two spatial dimensions. By reducing the experimental challenges as compared to existing architectures and by enabling room-temperature quantum computation, our design opens the door to scalable fabrication and operation, which may allow photonics to leap-frog other platforms on the path to a quantum computer with millions of qubits.

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Bonizzoni, C., A. Ghirri, K. Bader, J. van Slageren, M. Perfetti, L. Sorace, Y. Lan, O. Fuhr, M. Ruben, and M. Affronte. "Coupling molecular spin centers to microwave planar resonators: towards integration of molecular qubits in quantum circuits." Dalton Transactions 45, no. 42 (2016): 16596–603. http://dx.doi.org/10.1039/c6dt01953f.

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Altintas, Azmi Ali, Fatih Ozaydin, Cihan Bayindir, and Veysel Bayrakci. "Prisoners’ Dilemma in a Spatially Separated System Based on Spin–Photon Interactions." Photonics 9, no. 9 (August 30, 2022): 617. http://dx.doi.org/10.3390/photonics9090617.

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Having access to ideal quantum mechanical resources, the prisoners’ dilemma can be ceased Here, we propose a distributed quantum circuit to allow spatially separated prisoners to play the prisoners’ dilemma game. Decomposing the circuit into controlled-Z and single-qubit gates only, we design a corresponding spin–photon-interaction-based physical setup within the reach of current technology. In our setup, spins are considered to be the players’ logical qubits, which can be realized via nitrogen-vacancy centers in diamond or quantum dots coupled to optical cavities, and the game is played via a flying photon realizing logic operations by interacting with the spatially separated optical cavities to which the spin qubits are coupled. We also analyze the effect of the imperfect realization of two-qubit gates on the game, and discuss the revival of the dilemma and the emergence of new Nash equilibria.

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Hilaire, Paul, Edwin Barnes, and Sophia E. Economou. "Resource requirements for efficient quantum communication using all-photonic graph states generated from a few matter qubits." Quantum 5 (February 15, 2021): 397. http://dx.doi.org/10.22331/q-2021-02-15-397.

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Quantum communication technologies show great promise for applications ranging from the secure transmission of secret messages to distributed quantum computing. Due to fiber losses, long-distance quantum communication requires the use of quantum repeaters, for which there exist quantum memory-based schemes and all-photonic schemes. While all-photonic approaches based on graph states generated from linear optics avoid coherence time issues associated with memories, they outperform repeater-less protocols only at the expense of a prohibitively large overhead in resources. Here, we consider using matter qubits to produce the photonic graph states and analyze in detail the trade-off between resources and performance, as characterized by the achievable secret key rate per matter qubit. We show that fast two-qubit entangling gates between matter qubits and high photon collection and detection efficiencies are the main ingredients needed for the all-photonic protocol to outperform both repeater-less and memory-based schemes.

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Sabín, Carlos. "Digital Quantum Simulation of Linear and Nonlinear Optical Elements." Quantum Reports 2, no. 1 (March 4, 2020): 208–20. http://dx.doi.org/10.3390/quantum2010013.

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We provide a recipe for the digitalization of linear and nonlinear quantum optics in networks of superconducting qubits. By combining digital techniques with boson-qubit mappings, we address relevant problems that are typically considered in analog simulators, such as the dynamical Casimir effect or molecular force fields, including nonlinearities. In this way, the benefits of digitalization are extended in principle to a new realm of physical problems. We present preliminary examples launched in IBM Q 5 Tenerife.

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Abu-Nada, Ali. "Quantum computing simulation of the hydrogen molecular ground-state energies with limited resources." Open Physics 19, no. 1 (January 1, 2021): 628–33. http://dx.doi.org/10.1515/phys-2021-0071.

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Abstract In this article, the hydrogen molecular ground-state energies using our algorithm based on quantum variational principle are calculated. They are calculated through a simulator since the system of the present study (i.e., the hydrogen molecule) is relatively small and hence the ground-state energies for this molecule are efficiently classically simulable using a simulator. Complete details of this algorithm are elucidated. For this, a full description on the fermions–qubits and the molecular Hamiltonian–qubit Hamiltonian transformations, is given. The authors search for qubit system parameters ( θ 0 {\theta }_{0} and θ 1 {\theta }_{1} ) that yield the minimum energies for the system and also study the ground state energies as a function of the molecular bond length. Proposed circuit is humble and does not include many parameters compared with that of Kandala et al., the authors control only two parameters ( θ 0 {\theta }_{0} and θ 1 {\theta }_{1} ).

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Pedersen, Kasper S., Ana-Maria Ariciu, Simon McAdams, Høgni Weihe, Jesper Bendix, Floriana Tuna, and Stergios Piligkos. "Toward Molecular 4f Single-Ion Magnet Qubits." Journal of the American Chemical Society 138, no. 18 (April 27, 2016): 5801–4. http://dx.doi.org/10.1021/jacs.6b02702.

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Fataftah, Majed S., and Danna E. Freedman. "Progress towards creating optically addressable molecular qubits." Chemical Communications 54, no. 98 (2018): 13773–81. http://dx.doi.org/10.1039/c8cc07939k.

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Luigi Gentili, Pier. "Molecular Processors: From Qubits to Fuzzy Logic." ChemPhysChem 12, no. 4 (December 14, 2010): 739–45. http://dx.doi.org/10.1002/cphc.201000844.

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Drahi, David, Demid V. Sychev, Khurram K. Pirov, Ekaterina A. Sazhina, Valeriy A. Novikov, Ian A. Walmsley, and A. I. Lvovsky. "Entangled resource for interfacing single- and dual-rail optical qubits." Quantum 5 (March 23, 2021): 416. http://dx.doi.org/10.22331/q-2021-03-23-416.

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Today's most widely used method of encoding quantum information in optical qubits is the dual-rail basis, often carried out through the polarisation of a single photon. On the other hand, many stationary carriers of quantum information – such as atoms – couple to light via the single-rail encoding in which the qubit is encoded in the number of photons. As such, interconversion between the two encodings is paramount in order to achieve cohesive quantum networks. In this paper, we demonstrate this by generating an entangled resource between the two encodings and using it to teleport a dual-rail qubit onto its single-rail counterpart. This work completes the set of tools necessary for the interconversion between the three primary encodings of the qubit in the optical field: single-rail, dual-rail and continuous-variable.

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Porfyrakis, Kyriakos. "(Invited) N@C₆₀ and N@C₇₀ for Quantum Information Processing: Beyond Qubits." ECS Meeting Abstracts MA2022-01, no. 11 (July 7, 2022): 817. http://dx.doi.org/10.1149/ma2022-0111817mtgabs.

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Endohedral fullerenes such as N@C60, where a single atomic nitrogen is trapped inside the fullerene cage, have been proposed as qubit architectures due to the remarkably long relaxation times of their p-electron spins (T1 = 0.375 ms, T2 = 0.25 ms). Molecular quantum computers are still at the fringes of the field as recent developments have focused on other implementations such as superconducting qubits. However, molecular approaches present some advantages such as the ability to use chemical functionalization for scaling up qubit architectures. This, combined with continuous progress on miniaturization of electrodes via e-beam lithography and other techniques, means that molecular approaches will continue to be of interest. In this talk, I will review the field of fullerene-based quantum information processing. I will present progress on the synthesis, chemical functionalization and alignment of N@C60 and N@C70 in different matrices. Recently, we were able to align N@C60 and N@C70 derivatives in a liquid crystal matrix with ordering parameter Ozz = 0.61. With the aligned samples, we were able to achieve addressability of the available 4-electron spin levels in endohedral nitrogen by coherent manipulations. Furthermore, these functionalized molecules give rise to endohedral fullerene qudits: multi-level computational units alternative to the conventional 2-level qubits. Qudits offer a larger state space for encoding information and thus can offer enhancement of quantum algorithm efficiency. Indeed, we were able to demonstrate the first ever geometric phase using pulsed EPR; something that was first proposed over 30 years ago!

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Czarnik, Piotr, Andrew Arrasmith, Patrick J. Coles, and Lukasz Cincio. "Error mitigation with Clifford quantum-circuit data." Quantum 5 (November 26, 2021): 592. http://dx.doi.org/10.22331/q-2021-11-26-592.

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Achieving near-term quantum advantage will require accurate estimation of quantum observables despite significant hardware noise. For this purpose, we propose a novel, scalable error-mitigation method that applies to gate-based quantum computers. The method generates training data {Xinoisy,Xiexact} via quantum circuits composed largely of Clifford gates, which can be efficiently simulated classically, where Xinoisy and Xiexact are noisy and noiseless observables respectively. Fitting a linear ansatz to this data then allows for the prediction of noise-free observables for arbitrary circuits. We analyze the performance of our method versus the number of qubits, circuit depth, and number of non-Clifford gates. We obtain an order-of-magnitude error reduction for a ground-state energy problem on 16 qubits in an IBMQ quantum computer and on a 64-qubit noisy simulator.

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Jeong, Hyunseok. "Converting qubits." Nature Photonics 17, no. 2 (February 2023): 131–32. http://dx.doi.org/10.1038/s41566-022-01147-z.

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Pal, Amit Kumar, Philipp Schindler, Alexander Erhard, Ángel Rivas, Miguel-Angel Martin-Delgado, Rainer Blatt, Thomas Monz, and Markus Müller. "Relaxation times do not capture logical qubit dynamics." Quantum 6 (January 24, 2022): 632. http://dx.doi.org/10.22331/q-2022-01-24-632.

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Quantum error correction procedures have the potential to enable faithful operation of large-scale quantum computers. They protect information from environmental decoherence by storing it in logical qubits, built from ensembles of entangled physical qubits according to suitably tailored quantum error correcting encodings. To date, no generally accepted framework to characterise the behaviour of logical qubits as quantum memories has been developed. In this work, we show that generalisations of well-established figures of merit of physical qubits, such as relaxation times, to logical qubits fail and do not capture dynamics of logical qubits. We experimentally illustrate that, in particular, spatial noise correlations can give rise to rich and counter-intuitive dynamical behavior of logical qubits. We show that a suitable set of observables, formed by code space population and logical operators within the code space, allows one to track and characterize the dynamical behaviour of logical qubits. Awareness of these effects and the efficient characterisation tools used in this work will help to guide and benchmark experimental implementations of logical qubits.

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Aravena, Daniel, and Eliseo Ruiz. "Spin dynamics in single-molecule magnets and molecular qubits." Dalton Transactions 49, no. 29 (2020): 9916–28. http://dx.doi.org/10.1039/d0dt01414a.

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Yousefjani, Rozhin, and Abolfazl Bayat. "Parallel entangling gate operations and two-way quantum communication in spin chains." Quantum 5 (May 26, 2021): 460. http://dx.doi.org/10.22331/q-2021-05-26-460.

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The power of a quantum circuit is determined through the number of two-qubit entangling gates that can be performed within the coherence time of the system. In the absence of parallel quantum gate operations, this would make the quantum simulators limited to shallow circuits. Here, we propose a protocol to parallelize the implementation of two-qubit entangling gates between multiple users which are spatially separated, and use a commonly shared spin chain data-bus. Our protocol works through inducing effective interaction between each pair of qubits without disturbing the others, therefore, it increases the rate of gate operations without creating crosstalk. This is achieved by tuning the Hamiltonian parameters appropriately, described in the form of two different strategies. The tuning of the parameters makes different bilocalized eigenstates responsible for the realization of the entangling gates between different pairs of distant qubits. Remarkably, the performance of our protocol is robust against increasing the length of the data-bus and the number of users. Moreover, we show that this protocol can tolerate various types of disorders and is applicable in the context of superconductor-based systems. The proposed protocol can serve for realizing two-way quantum communication.

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Parrado-Rodríguez, Pedro, Ciarán Ryan-Anderson, Alejandro Bermudez, and Markus Müller. "Crosstalk Suppression for Fault-tolerant Quantum Error Correction with Trapped Ions." Quantum 5 (June 29, 2021): 487. http://dx.doi.org/10.22331/q-2021-06-29-487.

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Physical qubits in experimental quantum information processors are inevitably exposed to different sources of noise and imperfections, which lead to errors that typically accumulate hindering our ability to perform long computations reliably. Progress towards scalable and robust quantum computation relies on exploiting quantum error correction (QEC) to actively battle these undesired effects. In this work, we present a comprehensive study of crosstalk errors in a quantum-computing architecture based on a single string of ions confined by a radio-frequency trap, and manipulated by individually-addressed laser beams. This type of errors affects spectator qubits that, ideally, should remain unaltered during the application of single- and two-qubit quantum gates addressed at a different set of active qubits. We microscopically model crosstalk errors from first principles and present a detailed study showing the importance of using a coherent vs incoherent error modelling and, moreover, discuss strategies to actively suppress this crosstalk at the gate level. Finally, we study the impact of residual crosstalk errors on the performance of fault-tolerant QEC numerically, identifying the experimental target values that need to be achieved in near-term trapped-ion experiments to reach the break-even point for beneficial QEC with low-distance topological codes.

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Issah, Ibrahim, Mohsin Habib, and Humeyra Caglayan. "Long-range qubit entanglement via rolled-up zero-index waveguide." Nanophotonics 10, no. 18 (November 17, 2021): 4579–89. http://dx.doi.org/10.1515/nanoph-2021-0453.

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Abstract Preservation of an entangled state in a quantum system is one of the major goals in quantum technological applications. However, entanglement can be quickly lost into dissipation when the effective interaction among the qubits becomes smaller compared to the noise-injection from the environment. Thus, a medium that can sustain the entanglement of distantly spaced qubits is essential for practical implementations. This work introduces the fabrication of a rolled-up zero-index waveguide which can serve as a unique reservoir for the long-range qubit–qubit entanglement. We also present the numerical evaluation of the concurrence (entanglement measure) via Ansys Lumerical FDTD simulations using the parameters determined experimentally. The calculations demonstrate the feasibility and supremacy of the experimental method. We develop and fabricate this novel structure using cost-effective self-rolling techniques. The results of this study redefine the range of light-matter interactions and show the potential of the rolled-up zero-index waveguides for various classical and quantum applications such as quantum communication, quantum information processing, and superradiance.

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Picó-Cortés, Jordi, and Gloria Platero. "Dynamical second-order noise sweetspots in resonantly driven spin qubits." Quantum 5 (December 23, 2021): 607. http://dx.doi.org/10.22331/q-2021-12-23-607.

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Quantum dot-based quantum computation employs extensively the exchange interaction between nearby electronic spins in order to manipulate and couple different qubits. The exchange interaction, however, couples the qubit states to charge noise, which reduces the fidelity of the quantum gates that employ it. The effect of charge noise can be mitigated by working at noise sweetspots in which the sensitivity to charge variations is reduced. In this work we study the response to charge noise of a double quantum dot based qubit in the presence of ac gates, with arbitrary driving amplitudes, applied either to the dot levels or to the tunneling barrier. Tuning with an ac driving allows to manipulate the sign and strength of the exchange interaction as well as its coupling to environmental electric noise. Moreover, we show the possibility of inducing a second-order sweetspot in the resonant spin-triplet qubit in which the dephasing time is significantly increased.

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Lu, Wangjun, Cuilu Zhai, Yan Liu, Yaju Song, Jibing Yuan, and Shiqing Tang. "Berry Phase of Two Impurity Qubits as a Signature of Dicke Quantum Phase Transition." Photonics 9, no. 11 (November 9, 2022): 844. http://dx.doi.org/10.3390/photonics9110844.

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In this paper, we investigate the effect of the Dicke quantum phase transition on the Berry phase of the two impurity qubits. The two impurity qubits only have dispersive interactions with the optical field of the Dicke quantum system. Therefore, the two impurity qubits do not affect the ground state energy of the Dicke Hamiltonian. We find that the Berry phase of the two impurity qubits has a sudden change at the Dicke quantum phase transition point. Therefore, the Berry phase of the two impurity qubits can be used as a phase transition signal for the Dicke quantum phase transition. In addition, the two impurity qubits change differently near the phase transition point at different times. We explain the reason for the different variations by studying the variation of the Berry phase of the two impurity qubits with the phase transition parameters and time. Finally, we investigated the variation of the Berry phases of the two impurity qubits with their initial conditions, and we found that their Berry phases also have abrupt changes with the initial conditions. Since the Dicke quantum phase transition is already experimentally executable, the research in this paper helps to provide a means for manipulating the Berry phase of the two impurity qubits.

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Winpenny, Richard E P. "Quantum Information Processing Using Molecular Nanomagnets As Qubits." Angewandte Chemie International Edition 47, no. 42 (October 6, 2008): 7992–94. http://dx.doi.org/10.1002/anie.200802742.

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Atzori, Matteo, Alessandro Chiesa, Elena Morra, Mario Chiesa, Lorenzo Sorace, Stefano Carretta, and Roberta Sessoli. "A two-qubit molecular architecture for electron-mediated nuclear quantum simulation." Chemical Science 9, no. 29 (2018): 6183–92. http://dx.doi.org/10.1039/c8sc01695j.

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Lowe, Angus, Matija Medvidović, Anthony Hayes, Lee J. O'Riordan, Thomas R. Bromley, Juan Miguel Arrazola, and Nathan Killoran. "Fast quantum circuit cutting with randomized measurements." Quantum 7 (March 2, 2023): 934. http://dx.doi.org/10.22331/q-2023-03-02-934.

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We propose a new method to extend the size of a quantum computation beyond the number of physical qubits available on a single device. This is accomplished by randomly inserting measure-and-prepare channels to express the output state of a large circuit as a separable state across distinct devices. Our method employs randomized measurements, resulting in a sample overhead that is O~(4k/ε2), where ε is the accuracy of the computation and k the number of parallel wires that are "cut" to obtain smaller sub-circuits. We also show an information-theoretic lower bound of Ω(2k/ε2) for any comparable procedure. We use our techniques to show that circuits in the Quantum Approximate Optimization Algorithm (QAOA) with p entangling layers can be simulated by circuits on a fraction of the original number of qubits with an overhead that is roughly 2O(pκ), where κ is the size of a known balanced vertex separator of the graph which encodes the optimization problem. We obtain numerical evidence of practical speedups using our method applied to the QAOA, compared to prior work. Finally, we investigate the practical feasibility of applying the circuit cutting procedure to large-scale QAOA problems on clustered graphs by using a 30-qubit simulator to evaluate the variational energy of a 129-qubit problem as well as carry out a 62-qubit optimization.

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Simoni, Mario, Giovanni Amedeo Cirillo, Giovanna Turvani, Mariagrazia Graziano, and Maurizio Zamboni. "Towards Compact Modeling of Noisy Quantum Computers: A Molecular-Spin-Qubit Case of Study." ACM Journal on Emerging Technologies in Computing Systems 18, no. 1 (January 31, 2022): 1–26. http://dx.doi.org/10.1145/3474223.

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Classical simulation of Noisy Intermediate Scale Quantum computers is a crucial task for testing the expected performance of real hardware. The standard approach, based on solving Schrödinger and Lindblad equations, is demanding when scaling the number of qubits in terms of both execution time and memory. In this article, attempts in defining compact models for the simulation of quantum hardware are proposed, ensuring results close to those obtained with standard formalism. Molecular Nuclear Magnetic Resonance quantum hardware is the target technology, where three non-ideality phenomena—common to other quantum technologies—are taken into account: decoherence, off-resonance qubit evolution, and undesired qubit-qubit residual interaction. A model for each non-ideality phenomenon is embedded into a MATLAB simulation infrastructure of noisy quantum computers. The accuracy of the models is tested on a benchmark of quantum circuits, in the expected operating ranges of quantum hardware. The corresponding outcomes are compared with those obtained via numeric integration of the Schrödinger equation and the Qiskit’s QASMSimulator. The achieved results give evidence that this work is a step forward towards the definition of compact models able to provide fast results close to those obtained with the traditional physical simulation strategies, thus paving the way for their integration into a classical simulator of quantum computers.

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Huang, Haiqing, Irfan Ahmed, Ahmed Ali, Xin-wei Zha, Raymond Hon-Fu Chan, and Yanpeng Zhang. "Relations between the average bipartite entanglement and N-partite correlation functions." Laser Physics 32, no. 7 (May 20, 2022): 075201. http://dx.doi.org/10.1088/1555-6611/ac6e44.

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Abstract We study the relations between the average bipartite entanglement and the N-partite length of correlation. We show that the N-partite length of correlation can completely determine the average bipartite entanglement for two and three-qubits. For four-, five- and six-qubit systems, the N-partite correlation functions may not determine the average bipartite entanglement. These results are novel and promising for pure state.

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Ardavan, Arzhang, Alice M. Bowen, Antonio Fernandez, Alistair J. Fielding, Danielle Kaminski, Fabrizio Moro, Christopher A. Muryn, et al. "Engineering coherent interactions in molecular nanomagnet dimers." npj Quantum Information 1, no. 1 (December 8, 2015). http://dx.doi.org/10.1038/npjqi.2015.12.

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AbstractProposals for systems embodying condensed matter spin qubits cover a very wide range of length scales, from atomic defects in semiconductors all the way to micron-sized lithographically defined structures. Intermediate scale molecular components exhibit advantages of both limits: like atomic defects, large numbers of identical components can be fabricated; as for lithographically defined structures, each component can be tailored to optimise properties such as quantum coherence. Here we demonstrate what is perhaps the most potent advantage of molecular spin qubits, the scalability of quantum information processing structures using bottom-up chemical self-assembly. Using Cr7Ni spin qubit building blocks, we have constructed several families of two-qubit molecular structures with a range of linking strategies. For each family, long coherence times are preserved, and we demonstrate control over the inter-qubit quantum interactions that can be used to mediate two-qubit quantum gates.

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Yan, Ye-Ting, Chengsong Zhao, Zhen Yang, Da-Wei Wang, and Ling Zhou. "Quantum state transfer with cavity-magnonics nodes." Journal of Physics B: Atomic, Molecular and Optical Physics, August 3, 2022. http://dx.doi.org/10.1088/1361-6455/ac86b1.

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Abstract We put forward a proposal to construct a quantum network using the hybrid cavity-magnonics system as two nodes. At each node, a cascade of the quantum system consists of cavity-magnonics and magnonic-qubit interactions, and the quantum interface between the flying qubit and superconducting qubitis mediated by magnon. Considering the phase resulting from the distance between two nodes, we derive a master equation for two superconducting qubits and show that by adiabatically controlling the cavity-magnon coupling, perfect quantum state transfer between two qubits can be realized. We also consider the influence of intrinsic dissipation of the magnetic mode and the cavity mode. Under the unideal case, the design time-dependent cavity-magnonics couplings obtained in the deal case are still employed. Our results show that low intrinsic loss in the magnetic mode and the cavity mode is still welcome for the high fidelity of state transfer.

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Yoneda, J., W. Huang, M. Feng, C. H. Yang, K. W. Chan, T. Tanttu, W. Gilbert, et al. "Coherent spin qubit transport in silicon." Nature Communications 12, no. 1 (July 5, 2021). http://dx.doi.org/10.1038/s41467-021-24371-7.

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AbstractA fault-tolerant quantum processor may be configured using stationary qubits interacting only with their nearest neighbours, but at the cost of significant overheads in physical qubits per logical qubit. Such overheads could be reduced by coherently transporting qubits across the chip, allowing connectivity beyond immediate neighbours. Here we demonstrate high-fidelity coherent transport of an electron spin qubit between quantum dots in isotopically-enriched silicon. We observe qubit precession in the inter-site tunnelling regime and assess the impact of qubit transport using Ramsey interferometry and quantum state tomography techniques. We report a polarization transfer fidelity of 99.97% and an average coherent transfer fidelity of 99.4%. Our results provide key elements for high-fidelity, on-chip quantum information distribution, as long envisaged, reinforcing the scaling prospects of silicon-based spin qubits.

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Noiri, Akito, Kenta Takeda, Takashi Nakajima, Takashi Kobayashi, Amir Sammak, Giordano Scappucci, and Seigo Tarucha. "A shuttling-based two-qubit logic gate for linking distant silicon quantum processors." Nature Communications 13, no. 1 (September 30, 2022). http://dx.doi.org/10.1038/s41467-022-33453-z.

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AbstractControl of entanglement between qubits at distant quantum processors using a two-qubit gate is an essential function of a scalable, modular implementation of quantum computation. Among the many qubit platforms, spin qubits in silicon quantum dots are promising for large-scale integration along with their nanofabrication capability. However, linking distant silicon quantum processors is challenging as two-qubit gates in spin qubits typically utilize short-range exchange coupling, which is only effective between nearest-neighbor quantum dots. Here we demonstrate a two-qubit gate between spin qubits via coherent spin shuttling, a key technology for linking distant silicon quantum processors. Coherent shuttling of a spin qubit enables efficient switching of the exchange coupling with an on/off ratio exceeding 1000, while preserving the spin coherence by 99.6% for the single shuttling between neighboring dots. With this shuttling-mode exchange control, we demonstrate a two-qubit controlled-phase gate with a fidelity of 93%, assessed via randomized benchmarking. Combination of our technique and a phase coherent shuttling of a qubit across a large quantum dot array will provide feasible path toward a quantum link between distant silicon quantum processors, a key requirement for large-scale quantum computation.

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Montenegro La Torre, Carlos Renzo Misael, Yonny Yugra, and Francisco De Zela. "Relationship between entanglement and polarization in tripartite states." Journal of Optics, August 18, 2022. http://dx.doi.org/10.1088/2040-8986/ac8aab.

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Abstract Entanglement and polarization of pure, two-qubit systems are known to be constrained by the relationship C2+P2=1. Here, C stands for concurrence, a standard measure of entanglement, and P refers to the degree of polarization of either of the two, single-qubit subsystems. The above constraint may be understood as reflecting that a system cannot be in a pure state, if it is entangled with another one. In order to explore the connection between entanglement and polarization beyond two-qubit systems, we addressed states in which at least one of the parties has dimensionality greater than two. The case of pure, tripartite states offered itself as a timely candidate. We focussed on three qubits and studied the connection between one qubit's polarization and its entanglement with the other two taken together. We derived new constraints, akin to the above one, and submitted them to experimental tests using single photons entangled in polarization. The latter provided two qubits, each one attached to one photon. The third qubit was chosen to be the path (momentum) of one of the photons. Our experimental results confirmed the validity of the new constraints. Our theoretical results hold also for classical light. Possible experimental tests could be done with so-called structured light beams.

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Journal articles on the topic 'Molecular qubits'

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